1. Field of the Invention
The present invention relates generally to a method and apparatus for surface analysis of a recording surface. In particular, it relates to a method and apparatus for optimizing a piezoelectric contact detection sensor to provide a monotonic response with increasing asperity interference.
2. Background
In a conventional magnetic storage drive, an air bearing slider supports a magnetic transducer in close proximity to a relatively moving recording surface. The recording surface typically comprises a rigid disk coated with a layer of magnetic material applied by a method such as spin-coating or sputtering. Coated disks must be free of asperities to assure long-term reliability and the data integrity at the head to disk interface, since asperities can lead to undesirable slider-disk contact or "head crash".
Glide height testing is one means for assuring an asperity-free disk. A slider is flown over the recording disk at a height equal to or below the desired data head fly height to analyze impacts between the slider and the disk surface. The slider includes one or more piezoelectric sensors bonded to an upper surface facing away from the recording surface. Piezoelectric materials are used because they generate an electric charge in response to internal stress. As the slider experiences rigid body displacement and flexural deformation, the adjacent sensor responds by generating a charge signal which may be monitored.
A dominant practice in the art has been to monitor the low frequency piezoelectric signals corresponding to rigid body displacement and indicative of slider contact with large asperities on the disk surface. But as sliders decrease in size, magnetic transducers become vulnerable to relatively small asperities. Experience among those skilled in the art has shown a class of asperities (e.g. disk delaminations) that are too small to cause head crashes, yet large enough to result in slider-disk contact adversely affecting device reliability. This class of asperities generates high frequency vibrations in the test slider which cannot be detected adequately by conventional means.
The optimal sensitivity to small disk asperities is obtained by monitoring the high frequency vibrations of a test slider. Yet the high frequency components, or bending mode frequencies, of the response signal may vary greatly. Many modes display a non-monotonic response with increasing asperity interference height, i.e. the distance between the tip of an asperity and the minimum slider fly height. Non-monotonic modes indicate the occurrence of disk contact but provide no useful information about the size of the asperity causing contact.
The trend in recent years has been to produce storage systems having smaller sliders than the conventional "large" or "100%" sliders (e.g. 4 mm long by 3.2 mm wide). Reductions in slider size necessitate a corresponding reduction in test slider dimensions for equivalent compliance to the recording surface. This reduction results in a weaker piezoelectric signal and poor signal-to-noise (S/N) ratio. S/N ratio has also been shown to decrease with decreasing glide height. Thus, optimizing test slider sensitivity becomes increasingly important for smaller slider designs.
It is therefore desirable for a slider of predetermined size and fly height to identify one or more high frequency bending modes displaying monotonic behavior with increasing asperity interference. To that end, it becomes necessary to analyze the various bending mode frequencies individually. One method for isolating bending mode components is to electronically filter the high frequency signal generated by the piezoelectric sensor. But such filtering requires several filtering stages and becomes difficult with low signal to noise ratios.
One alternative to electronic filtering is to select a sensor design which facilitates the separation and/or detection of signal components of different frequencies. U.S. Pat. No. 4,532,802 describes a piezoelectric slider for isolating the low frequency components of a test slider signal. The apparatus comprises a slider having four piezoelectric transducers positioned on its upper surface facing away from the disk. Two sensors are positioned at the leading edge and two at the trailing edge. Independent examination of the low-frequency output signal from each transducer enables one to identify the components corresponding to pitch, roll, and vertical acceleration. This patent does not address analysis of high-frequency bending modes, and the head is both costly and complex to manufacture.
An IBM Technical Disclosure Bulletin article entitled "Efficient Piezoelectric Glide Transducer for Magnetic Recording Disk Quality Assurance", Vol. 34, No. 4A, Sep. 1992, describes a test slider comprising two piezoelectric transducers disposed on the upper surface of a slider about its longitudinal axis. Each half is oppositely poled with respect to the other. The arrangement increases the sensor's sensitivity to three low-frequency bending modes indicative of slider rigid body motion. Again, detection of the high frequency bending modes is not discussed.
U.S. Pat. No. 4,868,447 discloses a piezoelectric sensor comprising one, two or four layers of stacked laminae, each layer responsive to a different bending mode frequency. The disclosed apparatus does not teach the selection of a monotonic high frequency bending mode for small asperity detection in glide height testing.
What is needed, for a slider of predetermined size and fly height, is a method and apparatus for analyzing the high frequency components of a test slider response during small asperity contact to identify one or more optimal high frequency bending modes displaying monotonic behavior for increasing asperity interference height. Additionally, what is needed is a method for designing a piezoelectric sensor optimized to detect the identified mode. Sensor optimization must be balanced with a consideration of manufacturing complexity and costs.